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Wong" , Benjamin Coddington , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, v9fs@lists.linux.dev, linux-afs@lists.infradead.org, linux-btrfs@vger.kernel.org, ceph-devel@vger.kernel.org, codalist@coda.cs.cmu.edu, ecryptfs@vger.kernel.org, linux-erofs@lists.ozlabs.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-nfs@vger.kernel.org, ntfs3@lists.linux.dev, ocfs2-devel@lists.linux.dev, devel@lists.orangefs.org, linux-cifs@vger.kernel.org, samba-technical@lists.samba.org, linux-mtd@lists.infradead.org, linux-mm@kvack.org, linux-unionfs@vger.kernel.org, linux-xfs@vger.kernel.org, bug-gnulib@gnu.org, Jeff Layton Subject: Re: [PATCH v7 12/13] ext4: switch to multigrain timestamps Date: Tue, 19 Sep 2023 16:52:43 +0200 Message-ID: <4511209.uG2h0Jr0uP@nimes> In-Reply-To: <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> References: <20230807-mgctime-v7-0-d1dec143a704@kernel.org> <20230919110457.7fnmzo4nqsi43yqq@quack3> <1f29102c09c60661758c5376018eac43f774c462.camel@kernel.org> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" X-Rspamd-Server: rspam09 X-Rspamd-Queue-Id: 3C14E40003 X-Stat-Signature: hmdtctnnjwt6bc1h9beykgrafa31uyqo X-Rspam-User: X-HE-Tag: 1695135176-188737 X-HE-Meta: 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 Dn1K0obA DMgtEOkw0wxjOJKS6KikWyCdeAJvGj3q20M1jB3kC1QNZM9c8yn7EY4dOrmB+bmTqoiixnea3gYpTDPUKznPN9g+B52ljyA83pDs28CGiAZ6msjC++VN+tnLtOdc3wP5yd+uNAlFsaCsLui65J0Qj7k0Nit294SC0qeOVbJbZeQtJPwY8+pS8ZVzS3ec9oZ7B3ImqG3wMfsIAehhpfhxyrk96xf/rxSZvUEuSkEPC8FIFlPIU45gxNnDHcNe1BuqPNtdBBsa9JsYiG3w= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Jeff Layton wrote: > I'm not sure what we can do for this test. The nap() function is making > an assumption that the timestamp granularity will be constant, and that > isn't necessarily the case now. This is only of secondary importance, because the scenario by Jan Kara shows a much more fundamental breakage: > > The ultimate problem is that a sequence like: > > > > write(f1) > > stat(f2) > > write(f2) > > stat(f2) > > write(f1) > > stat(f1) > > > > can result in f1 timestamp to be (slightly) lower than the final f2 > > timestamp because the second write to f1 didn't bother updating the > > timestamp. That can indeed be a bit confusing to programs if they compare > > timestamps between two files. Jeff? > > > > Basically yes. f1 was last written to *after* f2 was last written to. If the timestamp of f1 is then lower than the timestamp of f2, timestamps are fundamentally broken. Many things in user-space depend on timestamps, such as build system centered around 'make', but also 'find ... -newer ...'. Bruno